184 FIRST FORMS OF VEGETATION. 
jected to the hypochlorite of lime test, the most 
beautiful reaction, while the older thallus is very 
little coloured. Nylander, one of the most emi- 
nent of our recent lichenists, proposes the use of 
the hypochlorite of lime test, as one of the easiest 
and most satisfactory modes of separating and 
distinguishing species of lichens which have been 
confounded by systematists. The reaction is pro- 
duced in many instances immediately upon the 
thallus being touched by the reactive ; and deter- 
minations perfectly exact can be made on speci- 
mens which are in a young and sterile state, and 
in other respects very incomplete. The least frag- 
ment is sufficient for the verification of the beauti- 
ful chemical character which distinguishes species 
in which other differences are scarcely visible. And 
there is this advantage to be gained by such a test, 
that it leads the student, by the differences which 
are manifested chemically, to search with more at- 
tention for organic characters, which as a rule will be 
found to accompany them. Asa striking example 
of the invaluable aid afforded by chemical reactives 
in the study of lichens, the case of the common yel- 
low wall-lichen, the Parmelia parietina, may be 
mentioned. This species is often confounded with 
young or sterile states of Lecanora candelarea, to 
which it has a remarkable resemblance. But a 
solution of hydrate of potash,—which is of equal 
